Rental fabric articles, for example, uniforms such as work clothes are cleaned every time they are returned and then rented again to supply. It is well known that the bar code system has been widely employed in the management of various items for sale. Thus, it is expected that the bar code system might be also applicable to the management of the rental clothes as described above.
A bar code-printed sheet in which bar code is heat transfer-printed on an ink-receiver layer which is a porous layer made of an ultra-high-molecular-weight polyethylene having a viscosity-average molecular weight of 300,000 to 10,000,000 and having an average pore size of 100 .mu.m or less is proposed in JP-A-9-39141 (the term "JP-A" as used herein means an unexamined published Japanese patent application).
This bar code-printed sheet has a number of advantages as follows: (a) Since the ultra-high-molecular-weight polyethylene per se has a high resistance to abrasion and has been impregnated with the ink held in the pores, the bar code-printed sheet has a remarkably high resistance to abrasion and thus withstands frequent wipe-off contacts encountering during transportation without resorting to over-coating. (b) Since the ink-receiver layer has a low heat conductivity, the heat energy required in the heat transfer printing is reduced and the heat amount accumulated in the heat transfer head is lowered, thus facilitating the heat transfer printing.
Accordingly, the present inventors have attempted to apply the bar code-printed sheet disclosed in the above JP-A-9-39141 to the management of rental fabric articles such as uniforms.
This bar code-printed sheet has a high resistance to abrasion so that the bar code can be read at a ratio of 100% even after rubbing with cloth moistened with artificial sweat. It is expected, in particular, that the resistance to abrasion would be elevated with a decrease in the surface roughness Ra. This is because a bar code-printed sheet having a lower surface roughness has the larger ink-contact area per unit area and thus the penetration of the ink into the porous layer is promoted and heat can be more uniformly transferred thereto from the heat transfer printing head, thus elevating the bond strength of the ink to the ink-receiver layer.
However, studies by the present inventors revealed that, even though a bar code-printed sheet having a porous layer made of the above ultra-high-molecular-weight polyethylene as an ink-receiver layer and having an average surface roughness Ra of 5 .mu.m or less was used, bar codes could not be successfully read with a bar code reader at a ratio of 100% in many cases after repeatedly washing (70.degree. C., 30 min) 100 times.
To clarify the reason therefor, the present inventors conducted further studies and, as a result, found out that the above-mentioned trouble was caused by the heat shrinkability of the ink-receiver layer.
The above-mentioned porous ink-receiver layer made of an ultra-high-molecular-weight polyethylene is produced by cutting a porous sintered product of the ultra-high-molecular-weight polyethylene powder into films or stretching an extrusion-molded ultra-high-molecular-weight polyethylene sheet. During the production process, the ultra-high-molecular-weight polyethylene molecular chains are solidified and immobilized in a forcibly extended state, which makes it unavoidable to make the ink-receiver layer thermally shrinkable. The maximum thermal stress f based on this heat shrinkability is expressed in the formula: EQU f=ke
wherein k means the degree of heat shrinkage, while e represents Young's modulus. Even though the ultra-high-molecular-weight polyethylene has a high Young's modulus, the porous structure has a considerably lowered Young's modulus. As a result, the above-mentioned thermal stress ke is lowered too.
In spite of such a low thermal stress, the dependency upon the heat shrinkability, which was revealed by the above studies, might be brought about by the fact that stress cracking in the ultra-high-molecular-weight polyethylene is accelerated by the contact with surfactants, alkalis, and solvents having high temperature (about 70.degree. C.) during frequent washing and thus the porous tissue of the ultra-high-molecular-weight polyethylene is broken.